The present invention relates to a new and improved optical measuring instrument for contactless measurement of the distance between the measuring instrument and an object.
In its more particular aspects, the present invention relates to a new and improved optical measuring instrument for contactless measurement of the distance between the measuring instrument and an object and comprising a radiation source which emits a coherent beam of radiation normally to the surface of the object. At least one radiation sensor element is mounted at the measuring instrument and receives scattered radiation reflected from the surface of the object which is located within a measuring range of the measuring instrument. An electronic evaluation circuit arrangement is connected to the output of the radiation sensor element. The position of the scattered light on the radiation sensor element which is received thereby through an optical device is a measure of the distance to be measured.
In an optical measuring instrument as known, for example, from U.S. Pat. No. 3,723,003, granted Mar. 27, 1973, the radiation sensor element comprises a series arrangement of a multiple number of photodiodes. The resolution of such optical measuring instrument is limited by the width of the photodiodes. This measuring instrument is unsuited for precision measurements with maximum precision.
A further optical measuring instrument as known, for example, from German Patent Publication No. 3,009,534, comprises a light source, two light sensor elements, each of which contains a multitude of individual sensors, and two optical devices, each of which projects onto the light sensor elements respective sections of two images which have been separately obtained from the object to be measured. In this arrangement there also exists the disadvantage that the resolution is restricted by the width of the individual sensors.
In a laser dimensional comparator as known, for example, from German Pat. No. 2,401,618, a laser light source directs coherent light normally to the surface of a workpiece to be measured in order to generate thereat a laser intensity gradient. An optical device is associated with a photo-detector in such a manner that a pair of images of the laser light-intensity gradient generated on the workpiece is imaged on the surface of the photo-detector. The images of the pair are spaced from each other and are reflected from the workpiece. The surface of the photo-detector is scanned and during the scanning operation two output pulses are generated which are separated in time by a time period which is proportional to the spacing which exists between the two image points of the light spot. The two output pulses switch scaling pulses which are generated by a scaling oscillator to a forward/backward counter. The scale or scaling pulses delivered to the counter are counted in a logic circuit and are compared to a reference value. In this arrangement the scanning frequency limits the resolution. This instrument furthermore has relatively large dimensions and is also economically disadvantageous.
In a further distance measuring instrument as known, for example, from German Pat. No. 2,650,422, a coherent light beam is directed to a measuring surface at a predetermined angle of incidence. The rays reflected from the measuring surface are guided to a receiver which contains a light sensor and which evaluates the angle of reflection for determining the distance. In front of the light sensor of the receiver there is arranged an involute-shaped slot diaphragm or stop which is located on a rotating disk, and thus, constitutes a movable measuring slit. The slot diaphragm or stop runs past a stationarily arranged apertured stop or diaphragm. The intersection point of the circulating slot diaphragm or stop and the stationary apertured stop or diaphragm defines the angular position of the circulating slot diaphragm or stop and thereby the angle between a ray, which is diffusely reflected from the measuring surface, picked up by a lens and passed through the slot diaphragm or stop, and the axis of the optical system and thus the distance under investigation.
In this arrangement the angular position of the circulating slot diaphragm or stop must be determined by counting a graduation at the rim or marginal portion of the disc, whereby the resolution of this instrument is also limited. In order to prevent a distance-dependent optical distortion, the rotating disc must be arranged parallel to the direction of the light ray which impinges on the object to be measured. As a result, the angle of incidence of the coherent light beam on the measuring surface is relatively small. It has been found by experience that false reflections of relatively high intensity occur in such an arrangement and result in a false measurement. The arrangement of the receiver in this instrument is such that all of the false reflections originating from the measuring surface reach the light sensor and result in the generation of a measuring signal. Therefore the uncertainty of the measurement is relatively great. The interfering effects can be counter acted at the light sensor by reducing the sensitivity thereof. Since, however, the intensity of the reflected light ray depends on the material of the measuring surface, the reduction in the sensitivity of the light sensor may have the result that measuring surfaces of poorly reflecting materials either cannot be utilized or require complicated adjusting operations.